Knowledge of the basic biology of the widespread, endemic respiratory pathogen, parainfluenza virus type 3 (para 3) is sparse and efforts to control it have been unsuccessful. There is evidence, moreover, that persistent infection (PI) of adults by this virus may contribute to its endemicity. In preliminary work, the Principal Investigator established stable PI by para 3 of a continuous line of cells in vitro (para 3 PI cells), utilizing para 3 that had been propagated by undiluted passage. It is proposed to investigate the mechanisms that initiated and maintain this system as an approach to the biology of para 3. Preliminary work suggests that defective interfering (DI) particles are involved. The nature of the DI particles in the para 3 PI cells will be elucidated using the electron microscope. Information will also be obtained by comparing the RNA species and polypeptides present in the para 3 PI cells with the RNAs and polypeptides present in para 3 virions and in cells lytically infected with para 3. Conditions leading to the formation of para 3 PI and/or DI particles in an appropriate animal host, viz., the hamster, will be examined. Results will clarify the biology of replication of para 3 and make possible approaches toward control of the agent.